As much as I'd love to see something along the lines of an integrated tracker to generate S files in the CC65 IDE, it's sadly difficult to do so in a standard way because FamiTracker 0.3.0, 0.3.5 & the latest betas are all different compatibility-wise. But considering that it's all open-source anyways, why not give it a shot?
Or, you can get an NES ROM disassembler & disassemble an NSF file (considering that NSFs are incomplete NES ROMs anyways).
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